In rodent brain, sexually dimorphic characteristics are organized by sex steroids (especially androgens and estrogens) during a discrete period in development called the "critical period" of sexual differentiation. In mice and rats, differentiative events which contribute to both acquisition of male-type characteristics (masculinization) and modulation of female-type characteristics (defeminization) begin prenatally. In mice and rats, putative receptors for both androgens and estrogens appear prenatally, suggesting that prenatal onset of their "critical period" may be a consequence of prenatal appearance of receptors, and that androgens as well a estrogens may directly mediate prenatal differentiation. To test the generality of these correllations, we will study hamsters since their "critical period" is primarily postnatal, and masculinization of females and defeminization of males do not occur to the extent seen in mice and rats. We will determine the temporal relationship between onset of the "critical period" and time of appearance of putative receptors for androgens, estrogens, and progestins in developing hamster brain. We will determine the appearance and subsequent ontogeny of each receptor in males versus females to reveal possible sex differences. The properties of receptors which will be examined at all ages (embryonic, neonatal, prepubertal, adult) include: DNA-binding; behavior during DNA-cellulose affinity chromatography and sucrose gradient sedimentation; hormone-binding affinity, specificity, and capacity; tissue specificity. We will also examine the temporal relationship between onset of the "critical period" and neuronal production in hypothalamic and limbic regions of hamster brain. In mice and rats, neurogenesis is prenatal and coincident with prenatal onset of their "critical period", suggesting that neuronal maturity may be another factor which temporally defines onset of the "critical period." By virture of its postnatal "critical period", the hamster again provides a model for testing this possibility. Using 3H-thymidine autoradiography, we will determine the time course of appearance of postmitotic neurons in specific regions of hasmter brain and investigate possible sex differences in both the temporal and spatial aspects of neurogenesis. By establishing the temporal relationship between ontogeny of sex hormone receptors, neuronal developmen, and onset of the "critical period," these studies may elucidate the feature(s) (biochemical, cellular, hormonal) intrinsic to postnatal hamster brain (male and female) which determine onset of sexual differentiation and mediate masculinization and defeminization.